Fielder hit two home runs in the game.
The Brewers victory, coupled with Chicago's loss at Florida, moved Milwaukee within two games in the NL Central race.
At 23 years old, Fielder hit home runs 49 and 50 to pass Willie Mays as the youngest player to hit at least 50 homers.
Jeff Suppan (11-12) got the win after allowing one run on nine hits over eight innings.
Braden Looper (12-12) suffered the loss, giving up seven runs on seven hits through five innings.